Yes, you have a point there. My stereo ENGLE cab can indeed set to mono.
But I have more cabs so I can create room between them. In my pre Powerhead life I did this a long time ago with 2 Mesa Boogie cabs and one of the speaker cables out of phase. Sounded amazing in stereo.
Anway, I have more possibilities with such a stereo power amp and my other stuff besides my Powerhead, like my Boss GT100...
Each to their own.
I have also previous run stereo in my ADA/Quadraverb days, with ping pong delays and enormous sound...which then promptly disappeared once the band joined in..
I swapped it out for a Laney GH100L, no reverb let alone effects.....cut through like a razor.
I have no doubt stereo can sound great and cut within a band context, but I would just consider:
1) What is the point of a Powerhead if you want to run stereo? So you could sell and just get a Stage. I have a Powerhead because of the power amp.
2) This is monitor only. You FOH can still be stereo. Yes, we all want to hear the best sound on stage but does it need to be exact
3) You bring in another variable. If valve, this will not come out in FOH, defeating point 2
4) Extra set up time, cabling, cost etc
5) You mentioned you did stereo across 2 cabs before, this is different than within one cab where the separation is inches. Separate cabs allow you to get better separation and therefore stereo.